1. Field of the Invention
The present invention relates generally to sleeved braces, or “buckling restrained braces,” and methods for manufacturing the same. More specifically, the present invention relates to buckling restrained braces that include yielding core members that extend through an outer sleeve which contains a buckling constraining material, which yielding core members are laterally spaced apart from the buckling constraining material by way of an air gap. Among other purposes, the buckling restrained braces of the present invention are useful in the construction of earthquake resistant structures, such as earthquake resistant steel building frames.
2. Background of Related Art
In order to understand the importance of the buckling restrained braces of the present invention, it is beneficial to briefly describe the nature of the forces that act on a building or other structure during an earthquake.
During an earthquake, the ground on which a building or other structure is built or by which the building or other structure is supported is subjected to a variety of primary vibratory motions, including vertical motion (i.e., up and down motion), lateral drift, inverted pendulum movement in one or more vertical planes, and plan rotation.
With reference to FIG. 1a, the framework of a typical multistory building, which comprises beams and columns, is shown. During the up and down vibratory motion of the ground, the whole building moves up with a vertical acceleration, as shown in FIG. 1b, and then, after reaching a peak, will move downward with a vertical acceleration as shown in FIG. 1c. This motion repeats during the duration of the earthquake. As the ground moves up and down, so does the building and its framework. Due to its mass, as the building accelerates vertically, its framework is subjected to additional vertical loads, depending on the direction of motion, as shown by the arrows in FIGS. 1b and 1c. The beams and columns of the framework of the building can be designed easily to withstand these additional vertical loads.
As the ground drifts laterally, the whole building will move laterally, with acceleration to one side, as shown in FIG. 1d, and, after reaching a peak value of drift, will move in the opposite direction, as shown in FIG. 1e. Because of the mass of the building and the lateral acceleration, the building frame will be subjected to cyclical lateral loads F1, F2, and F3, as shown by the arrows in FIG. 1d and FIG. 1e. These lateral loads may result in severe damage to the framework of the building. Conventionally, to counteract lateral loads, complex framework designs have been developed, their complexity making them somewhat undesirable and often increasing the costs associated with erecting the framework of the building.
Inverted pendulum motion of the ground causes the entire framework of a building and, thus, the entire building, to rotate in a vertical plane with an angular acceleration. Once a peak value of rotation has been reached, the building and its framework will rotate in the reverse direction. During such angular acceleration, and due to the mass of the building, the building frame will be subjected to additional cyclical lateral loads F1, F2, and F3, as shown by the arrows in FIG. 1f and FIG. 1g. 
During plan rotation of the ground, the building will rotate in plan with an angular acceleration and, after reaching a peak rotation, will rotate in the reverse direction. Because of the mass of the building and the angular acceleration, lateral forces will act on the frame, as shown by the arrows in FIGS. 1h and 1i. 
Many design procedures are available to design the building framework that can withstand these earthquake-induced additional lateral loads. In this context, it is mentioned that many codes of practice in the United States recommend that the building framework remain elastic, or nearly so, under moderate earthquakes of frequent occurrence, but be able to yield locally without serious consequences during major earthquakes.
Many types of structural frame configurations and designs that are intended to resist earthquake-induced loads are presently available.
For example FIG. 2a shows a normal building frame comprising beams 1 and columns 2. The beams 1 are supported on seating cleats 3 that are located on and secured to the columns 2. The columns 2, in turn, are supported on base plates 4. By avoiding the inclusion of diagonal members, each opening, or “bay,” between adjacent pairs of beams 1 and columns 2 readily accommodates doors, windows, service ducts, and the like. Without diagonal members, however, when subjected to earthquake (i.e., seismic) or other loads, the frame undergo excessive lateral sway, or drift, as shown in FIG. 2b, when lateral forces F1, F2, and F3 act thereon. In order to counteract loads and, thus, reduce or prevent such excessive lateral sway, the connections between the beams 1 and columns 2 are made rigid.
FIG. 3a shows a rigid frame design which includes beams 5, columns 6, stiffeners 7 positioned proximate the junction of each beam 5 with a column 6, and base plates 8 located at the bottom ends of columns 6. The end of beam 5 is connected to the flange of column 6 by a full-strength weld. Stiffeners 7 are welded to the column 6 to prevent the flange of each column 6 from bending outwardly. Additionally, a plastic hinge may be positioned adjacent to each beam 5-to-column 6 junction. FIG. 3b shows an enlarged view of the rigid connection between a beam 5 and a column 6 of the rigid frame design of FIG. 3a. FIG. 3c is a cross-sectional representation taken along line A—A of FIG. 3b. 
This configuration of moment-resisting frame is able to resist the lateral forces F1, F2, and F3 and exhibits low stiffness and high ductility, which are desirable features in earthquake-resistant structural systems. FIG. 3d shows the deflected shape of the frame when subjected to earthquake-induced lateral forces F1, F2, and F3. When the frame is subjected to an earthquake-induced load, some of the energy is dissipated at the plastic hinge. Frequently, this system suffers severe drift as well as premature failure at the beam 5-to-column 6 connections, which may render it non-functional even after moderate earthquakes. Further, this system is not viable for tall buildings.
FIG. 4a shows a frame with concentric “tension only” intersecting diagonal bracings 12 and 13. The frame includes columns 11, beams 10, and diagonal bracings 12 and 13. The diagonal bracings 12 extend in the direction labeled as “X.” The diagonal bracings 13 extend in the direction labeled as “Y.” The diagonal bracings 12 and 13 typically include rolled steel angle sections. The diagonal bracings 12 and 13 cross each other and, hence, are also referred to as “intersecting diagonals,” which are arranged as an “X” in each bay formed by adjacent pairs of columns 11 and beams 10. A base plate 17 is positioned at the bottom, or base, of each column 10. An end plate 14 is welded to the end of each beam 10 and, thus, abuts the column 11 when the beam 10 is positioned adjacent thereto. Gusset plate 15, 16 are secured at the junctions between each column 11 and beam 10 to facilitate the securing of a diagonal bracing 13, 12, respectively, to the remainder of the frame. In actual practice, the gusset plates 15 may have a different size than gusset plates 16, which sizes depend on the force in the diagonal bracing 13, 12, respectively, to be secured thereto.
FIG. 4b shows the joint between each column 11, beam 10, end plate 14, diagonal bracing 12, 13, and gusset plate 15, 16. Again, the beam 10 has an end plate 14 welded to an end thereof. The end plate 14 has holes to facilitate connection thereof and, thus, of the beam 10, to the column 11. The flange of the column 11 has matching holes for connecting to end plate 14. Gusset plates 15, 16 are welded to both a beam 10 and an end plate 14. Diagonal bracings 13, 12 are respectively secured to the gusset plates 15, 16 by bolts. In this connection, the centerlines of column 11, beam 10, and diagonal bracings meet at point “a” and, hence, the bracing is referred to as “concentric.” In this design, the tension diagonals 12 and 13 are very slender and can resist tension well, but buckle under even little compressive force.
As shown in FIG. 4c, F1, F2, and F3 represent earthquake-induced lateral loads that act on the frame at different floor levels. When earthquake induced lateral forces F1, F2, and F3 act at each floor level of the frame in the direction of the arrows, as shown in FIG. 4c, the frame will deflect laterally, as shown, and the diagonal bracings 12 will be subjected tension, while the diagonal bracings 13 will buckle under slight compressive force. When the direction of loading reverses, as shown in FIG. 4d, diagonal bracing 13 will be in tension and diagonal bracing 12 will buckle and become ineffective, as shown.
This system resists the earthquake induced lateral loads very effectively because of the presence of diagonals in the framework. The connection details are also quite simple. If, during a severe earthquake, the tension in the diagonal bracings 12, 13 exceeds their yield strength, they enter a plastic state and absorb shock energy well. However, they will become permanently elongated. Under repeated cyclic loading, both the diagonal bracings 12 and 13 undergo larger permanent elongation and, as a result, the structure degrades. Once the structure degrades, the lateral drift of the frame will be beyond acceptable limits, even in minor earthquakes.
A frame that includes diagonal bracing which is configured to absorb both tension and compression is shown in FIGS. 5a–5d. Such a frame includes beams 18, columns 19, diagonal bracing 20, and end plate 21 at the end of each beam 18, and a gusset plate 22 secured to a beam 18 and an end plate 21 at the junction between that beam 18 and a column 19. In addition, a base plate 23 is secured to the bottom, or base, of each column 19.
The junction between a beam 18, column 19, and diagonal bracing 20 is shown in FIG. 5b. The centerlines of beam 18, column 19, and diagonal bracing 20 meet at point “g” and, hence, the bracing is said to be “concentric.”
As depicted in FIG. 5c, when lateral loads F1, F2, and F3 are exerted on the frame in the directions of the arrows, the diagonal bracing 20 will be compressed. When the direction of loading reverses, as shown in FIG. 5d, the same diagonals will be in tension.
In such a brace design, when a diagonal bracing 20 is in tension, it will undergo plastic deformation when subjected to load beyond its yield strength and absorb shock energy. However, when the same diagonal bracing 20 is compressed, it will buckle at a far lesser load without absorbing any shock energy. In order to prevent premature buckling, it is necessary to increase the stiffness of each diagonal bracing 20 by adopting a much larger structural section. This makes the diagonal bracing 20 very heavy and expensive. Although the lateral drift of a building including such a frame is significantly reduced, providing a very stiff diagonal bracing increases the total stiffness of the frame which, in turn, generates larger lateral shears (loads) at the foundation level of the building, which is not desirable. Also, when the diagonal bracings 20 are subjected to a compressive force beyond their yield strengths, they will buckle suddenly without absorbing much energy.
The so-called “eccentric bracing system,” illustrated in FIG. 6, is a design which improves upon the preceding frame designs and which has been extensively adopted across the world. Like the previously-described frame designs, an eccentric bracing system includes beams 24, columns 25, and diagonal bracings 26 and 27. Diagonal bracing 26 is secured within a bay between two beams 24, while one end of diagonal bracing 27 is secured in a vertically adjacent (e.g., next-lower, as shown) bay to a beam 24, with the other end of diagonal bracing 27 being secured to a column 25. Additionally, an end plate 28 is secured to an end of each beam 24. The end plate 28 has holes formed therethrough to facilitate securing the beam 24 to which it is secured to a column 25. Gusset plates 29, which include holes therethrough to facilitate the securing of corresponding ends of a diagonal bracing 26 thereto, are secured to opposed surfaces of the beams 24 that form the top and bottom of a bay within which the diagonal bracing 26 is located. Another gusset plate 31 is positioned at the junction between a column 25 and a base plate 30 that has been secured to the bottom, or base, of the column 25. The gusset plate 31 includes holes to facilitate securing of a lower end of a diagonal bracing 27 thereto, the opposite, upper end of the diagonal bracing 27 being secured to a beam 24 by way of a gusset plate 29 protruding from the bottom of the beam 24.
It can be seen in FIG. 6 that the centerline of diagonal bracing 26 and the centerline of beam 24 meet at point “k”, whereas the centerline of column 25 and the centerline of beam 24 meet at point “h”. Thus there is an eccentricity of ‘e1’ (i.e., the distance h–k).
Eccentric bracing systems are not as stiff as concentric bracing systems. Under severe seismic load, a hinge in the beam is formed at point “k”, leading to dissipation of considerable energy. However, due to severe plastic hinge deformation of the beam link at point “k”, frames which employ eccentric bracing systems suffer from considerable drift, even under loads applied thereto by moderate earthquakes. Moreover, repairing the shock-absorbing capabilities of eccentric bracing systems is very expensive.
According to a report published in 1988, Nippon Steel Company, has developed a so-called “unbonded brace” for use as a diagonal bracing in earthquake-resistant building frames. FIGS. 9a–9f depict an example of such an unbonded brace 48, while FIGS. 10a–10c show use of that unbonded brace 48 in a building frame.
As shown in FIGS. 9a–9f, unbonded brace 48 includes a yielding core 41, a flexible coating of “unbending material” 42 that surrounds the yielding core 41, grout 44 surrounding the yielding core 41 and the unbonding material 42, and a hollow steel sleeve 43 which contains the grout 44, the unbonding material 42, and a substantial portion of the length of the yielding core 41. The core 41, which is depicted, without limitation, as having a rectangular cross-section, includes coupling ends 45, or “plus sections,” that are provided with holes to facilitate securing of the coupling ends 45 and, thus, of the yielding core 41 of the unbonded brace 48 to corresponding gusset plates that have been secured to a frame of a building.
A hollow pocket S having a length L1 remains at both ends of the grout 44 so that the coupling ends 45 of the yielding core 41 will not collide with and, thus, impact the grout 44 as the yielding core 41 is compressed. Each pocket S is filled with flexible polystyrene 46.
The unbending material 42, which has a length L2 along a central section of the yielding core 41 ensures that the grout 44 does not bind to the yielding core 41 and that an axial load on the yielding core 41 is not transferred to the grout 44 or to the sleeve 43. Thus, the axial load is resisted only by the yielding core 41.
The grout 44 and the sleeve 43, by the virtue of their flexural stiffness, prevent lateral buckling of the yielding core 41.
As shown in FIG. 10a, the unbonded brace 48 has been used as a diagonal bracing in earthquake-resistant building frames to control lateral drift thereof and also to absorb energy which is transferred to such frames. A building frame fitted with this unbonded brace 48 also includes columns 46 and beams 47. The unbonded brace is secured to the frame, proximate to junctions between the columns 46 and beams 47, by way of gusset plates 49 that have been secured to a column 46 and a beam 47 at a junction thereof.
FIG. 10b shows the earthquake-induced lateral loads F1, F2, and F3, which act in the directions of the illustrated arrows. Under this loading, the unbending brace 48 will be in tension. The yielding core 41 of the unbonded brace 48 will resist this tension and has the capacity to absorb energy when subjected to a tensile force beyond the yield strength thereof. Thus, substantial energy will be absorbed during severe earthquakes. The lateral drift is also controlled.
FIG. 10c shows the reversed earthquake-induced lateral loads F1, F2, and F3 acting in the directions of the corresponding depicted arrows. Under this loading, the unbonded brace 48 is in compression. Then the yielding core 41 of the unbonded brace 48 will start to buckle, but the grout 44 and the sleeve 43 will prevent the yielding core 41 from buckling. The yielding core 41 can absorb significant energy, even under compressive force, when loaded beyond its yield strength during a severe earthquake.
One of the drawbacks of the Nippon Steel Company unbending brace 48 is the potential for damage to and/or degradation of the unbonding material 42 over the course of time or following tension and/or compression of the yielding core 41 of such an unbending brace 48. If the unbonding material 42 degrades or becomes damaged, friction will develop between the yielding core 41 and the grout 44. As a consequence, axial loading of the yielding core 41 will be undesirably transferred to the grout 44 and the sleeve 43.
Moreover, the flexible polystyrene 46 used in such unbending braces 48 is not fully fire resistant. Nor, as shown in FIG. 11a, can the flexible polystyrene 46 be relied upon to provide sufficient lateral support to the thin yielding core 41. While unbending brace 48 works well provided the axial force acting on the yielding core 41 is concentric, i.e., center lines through the unbonding brace 48, the beam 47, and the column 46 intersect at a single point. If there is an eccentricity “e2” due to fabrication deviations, then the yielding core 41 will no longer be carrying purely axial load, but will be subjected to a bending moment Ml equal to the axial force F3 multiplied by the eccentricity “e2”. Consequently, the yielding core 41 may bend in the gap L1, as shown in FIG. 11b. This bending of the yielding core 41 will cause premature failure of the unbending brace 48. Furthermore, the unbending brace 48 is rigidly connected to the building frame with several bolts instead of a single pin joint. This type of multiple bolted connection causes secondary moments on the yielding core 41. This secondary moment M also causes the core to bend, as shown in FIG. 11b. Also the grout 44 will be generally of considerable self weight and due to lateral acceleration of the building during a severe earthquake, this self weight of grout itself generates lateral forces and bending moments on the thin yielding core 41. Furthermore, during a severe earthquake, the cladding materials like bricks, tiles etc., may loosen first and fall on the bracing member. This falling debris may also result in bending of the yielding core 41 within the gap L1.
Another drawback of the Nippon Steel Company unbonded brace 48 is that if it is to be long for use in a large structure, then the axial deformation of the yielding core 41 will also be very large. Hence, the gap L1 (FIG. 9a) will also have to be large. Here again, as the brace tends to be very heavy due to the weight of the grout therein, problems may occur due to local buckling of the yielding core 41 in the gap L1.
In the United States, The American Institute of Steel Construction (AISC) has published specifications for the design of steel structures. Their specifications are widely followed by design engineers. A committee of AISC has prepared a draft specification for buckling restrained braces which is likely to be incorporated, as an appendix, into the AISC Code of Practice. The draft specification specially mentions that the bracing member should be capable of resisting any bending moment and lateral forces caused are eccentricity of connections and other factors.
The unbonded bracing system of Nippon Steel Company uses the basic principles that have been disclosed in Indian Patent No. 155036, for which an application was filed on Apr. 30, 1981 (hereinafter “the Indian Patent”), and in U.S. Pat. No. 5,175,972, issued Jan. 5, 1993 (hereinafter “the '972 patent”). Each of these systems includes a yielding core and a sleeve to restrain the yielding core from buckling.
The column of the Indian Patent is depicted in FIGS. 7a and 7b and includes a tubular sleeve 32 having a circular cross-section and a core rod 33 housed inside the sleeve 32. A gap of predetermined distance separates the core rod 33 from the sleeve 32. The Indian Patent also discloses that “[t]he sleeve can be isolated from the core by providing rubber washers with the result that performance is better under vibratory conditions.” A first end of the core rod 33 extends a predetermined distance beyond the corresponding first end of the sleeve 32. In addition, the column of the Indian Patent is described as including a base plate 34 secured to the second end of the sleeve 32.
In addition, FIG. 7a depicts the application of an axial load W to the core rod 33. The column shown in FIG. 7a supports the axial load W in the following manner: The load W is resisted only by the core rod 33, not by the sleeve 32. Without the presence of sleeve 32 surrounding the core rod 33, the load W that has been applied to the core rod 33 will cause the core rod 33 to buckle. However, since the sleeve 32 surrounds much of the core rod 33, the core rod 33 will come in to contact with the inside surface of the sleeve 32 which, by virtue of its flexural stiffness, will prevent any further lateral buckling of the core rod 33. Thus, the core rod 33 alone supports the entire load and the sleeve 32 acts merely as a buckling restraining member. Accordingly, with this arrangement, it is possible to load the core rod 33 beyond its yield strength and to cause it to absorb energy by providing a surrounding sleeve 32 with suitable flexural stiffness.
FIGS. 8a and 8b depict the scaffolding prop that is described in the '972 patent. That scaffolding prop includes a plurality of core rods 35, 36 that have been placed, end-to-end, inside a hollow sleeve 37, with a small, predetermined annular gap therebetween. One long core rod can be used in place of the plurality of core rods 35, 36.
The uppermost core rod 36, which protrudes beyond the sleeve 37, has threads 38 at an upper end thereof to facilitate securing thereof to a socket 38 that is associated with a roof slab 40 of a building that is supported by the scaffolding prop. The socket 38 does not contact the edge of the sleeve 37. A base plate 39 is rigidly secured to a bottom end, or base, of the sleeve 37. The bottom-most core rod 35 rests freely on the base plate 39.
The scaffolding prop of FIG. 8a supports the load of the roof slab in the following manner: the weight of the roof slab 40 is transferred to the ground, sequentially, through the socket 38, the core rods 36, 35, and the base plate 39. Without the sleeve 37, the core rods 35, 36, would buckle when subjected to a compressive load due to the weight of the roof slab 40. The sleeve 37, however, prevents such buckling. In particular, when a compressive load is applied to the core rods, 35, 36, the sides thereof will contact the inside surface of the sleeve 37 and the sleeve 37, by the virtue of its flexural stiffness, will prevent the further lateral buckling of the core rods 35, 36. Thus, the core rods 35, 36 will absorb the majority of the load placed thereon. The sleeve 37 acts primarily as a buckling restraining member. Thus, it is possible, by giving suitable flexural stiffness to sleeve 37, to load the core rods 35, 36 beyond their collective yield strength, allowing them to absorb shock energy.
During earthquakes in Kobe, Japan, San Francisco, Calif., and Turkey, many buildings were totally destroyed, even though many of them had been designed with frames that incorporated the foregoing systems.
There is, therefore, an urgent need to develop a safer, more effective bracing system.